1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for steerable beam antennas, and more particularly to array structures that can be used for steering antenna beams.
2. Description of the Related Art
Beam steering relates to changing the direction of a beam emitted from an antenna. In general, there are two ways to steer the beam of an antenna: mechanically and electrically. A mechanically scanned antenna uses a gimbal or other device to physically change the direction the antenna is pointing. An electrically steered antenna uses an array of elements onto which a progressive phase shift is applied. This progressive phase shift along the array determines the direction from which the energy will add in phase when summed by the array. It will be appreciated that these structures are generally reciprocal in their operation. If the system will steer a transmitted beam, it will also steer the received beam in a similar way.
There are several ways to make reflective structures perform a beam steering function. A simple flat conducting plate can be used to perform beam steering by moving the orientation of the reflector plate. The same effect can be achieved using a flat conductive plate covered by a uniform dielectric. However, it is often desirable to steer the beam without the need for gross mechanical movement of the reflector plate. Conventional reflectarrays can perform this function electronically. One example of a reflectarray is disclosed in U.S. Pat. No. 4,684,952 to Munson et al. However, alternative arrangements are also known in the art.
A frequency selective surface (FSS) is conventionally designed to either block or pass electromagnetic waves at a selected frequency, but is not generally used for beam steering. These types of surfaces are generally comprised of a conducting sheet periodically perforated with closely spaced apertures, or an array of periodic metallic patches. One or more layers formed with such structures can be used in the FSS.
FSS arrays that use metallic patches are sometimes called wire arrays. The patches can be formed in a dipole configuration or can have more than two-fold symmetry. For example, the patches can be tripoles or quadrupoles. FSS arrays that use openings in an otherwise continuous conductive sheet are commonly referred to as slot arrays. Slot and wire arrays can be combined in a single FSS having multiple layers. Optionally, a ground plane may be included as one of the layers of the FSS if the surface is to have reflective properties.
In general, an FSS comprised of wire arrays will be reflective in some frequency band (called the stopband) and transmissive at other frequencies. Conversely, slot arrays are generally transmissive in some frequency band (called the passband) and reflective at other frequencies. The phase shift caused by the FSS with respect to reflected or transmitted waves varies significantly in the transition region between passband and stopband. An FSS over a ground plane is of course entirely reflective, but the phase of the reflected wave will be different in the passband and stopband.
Many types of FSS element configurations are known, including circles, Jerusalem crosses, dipoles, tripoles, quadrupoles, concentric rings, mesh-patch arrays or double squares supported by a dielectric substrate. Depending upon the geometry selected, these can combine features of inductive and capacitive elements and can be used to provide low-pass, high-pass, or band-pass responses. U.S. Pat. No. 3,231,892 describes some basic FSS geometries and one potential application for an FSS type periodic resonance structure. Notably, signals that are blocked by an FSS are typically reflected away from the FSS, but the reflected direction is often not a matter of concern for the designer.
Properties of the FSS, such as frequency response, are determined by element shape in arrays, element spacing in arrays, properties of dielectric materials comprising the FSS, and the presence or absence of a ground plane. With regard to the dielectric, it is known that FSS properties are significantly affected by the permittivity/permeability of the material in close proximity to the array layers.